Semi-Persistent Scheduling (SPS) is a new scheduling method proposed in 3G Long Term Evolution (LTE) to save a Physical Downlink Control Channel (PDCCH), and was proposed primarily for a Voice over IP (VoIP) service. A general idea of the SPS lies in that a newly transmitted packet of the VoIP service arrives at an interval of 20 ms, thus a periodicity of reserved resources may be indicated through Radio Resource Control (RRC) signaling, then reserved resources in the time and frequency domains may be activated over a Physical Downlink Control Channel (PDCCH), and subsequently data may be transmitted automatically over resources at a fixed location at the interval of 20 ms without indicating the allocated resources for each newly transmitted packet over the PDCCH; and that resources to be occupied by a retransmitted packet can not be reserved but have to be scheduled dynamically due to unpredictability thereof. In view of this, it is referred to as semi-persistent scheduling as illustrated in FIG. 1.
In an LTE Time Division Duplex (TDD) system, there are seven proportional configurations of uplink and downlink sub-frames, respectively Configurations 0 to 6, for five of which Round Trip Time (RTT) of a Hybrid Automatic Repeat reQuest (HARQ) corresponding to uplink transmission is 10 ms. Since the uplink of TD-LTE (i.e., TDD LTE) is based upon a synchronous non-adaptive HARQ, that is, the same resources are occupied and the same transmission format is adopted for a retransmitted packet as a newly transmitted packet (i.e., an initially transmitted packet) in the case that there is no indication over the PDCCH, the HARQ packet transmitted for the second time may conflict with resource allocation of the semi-persistent scheduling for the newly transmitted packet. As illustrated in FIG. 2, reference numbers 1, 2 and 3 in FIG. 2 represent serial numbers of uplink synchronous HARQ processes respectively (a newly transmitted packet and its retransmitted packet correspond to the same serial number of an HARQ process), and as can be apparent, if both of the uplink HARQ processes 1 and 2 are used to transmit data of the same UE, the same resources will be occupied by a retransmitted packet of the uplink HARQ process 1 and a newly transmitted packet of the uplink HARQ process 2 after elapsing of 20 ms since a newly transmitted packet of the uplink HARQ process 1 is transmitted, thus causing resource confliction.
In order to address the problem of resource confliction between a retransmitted packet and a newly transmitted packet in the TD-LTE semi-persistent scheduling, a solution referred to as semi-persistent scheduling in a multi-periodicity mode has been proposed. A semi-persistent scheduling periodicity (i.e., an interval for resource allocation) applicable to a VoIP service is typically 20 ms, while there are two periodicities for the semi-persistent scheduling in the multi-periodicity mode, i.e., T1 and T2, where T1+T2=40 ms and T1 and T2 are active alternately. The relationship between T1 and T2 may be as follows.T1=SPS periodicity+delta  (1)T2=SPS periodicity−delta  (2)
Where the SPS periodicity represents a periodicity of the semi-persistent scheduling, which is 20 ms for the VoIP service, and the delta represents an offset of the periodicity of the semi-persistent scheduling.
As proposed in an existing solution, the value of delta in Equations (1) and (2) may be specified dependent upon a configuration of TD-LTE uplink and downlink sub-frames and a location of an uplink sub-frame at the beginning of semi-persistent scheduling in a TDD periodicity, that is, the value of delta may be determined uniquely when the semi-persistent scheduling starts with a specific uplink sub-frame, and RRC signaling is not necessary for notifying a User Equipment (UE) of the value of delta to be used, while 1-bit RRC signaling is necessary for indicating whether to use the semi-persistent scheduling in the multi-periodicity mode. For example, in the case of TDD Configuration 2, there is one uplink sub-frame in each TDD periodicity of 5 ms, and according to this solution, the value of delta is calculated in the following equation.Delta=1+number of DL sub-frames  (3)OrDelta=−1−number of DL sub-frames  (4)
Where the number of DL sub-frames in Equations (3) and (4) represents the number of downlink sub-frames in a 5 ms TDD periodicity (where a special sub-frame may be regarded as a downlink sub-frame due to transmission of downlink data therein), which is four for the VoIP service in the case of TDD Configuration 2, therefore the value of delta corresponding to any semi-persistent scheduling in the multi-periodicity mode starting with an uplink sub-frame is 5 ms or −5 ms.
FIG. 3 illustrates a schematic diagram of semi-persistent scheduling in the multi-periodicity mode, and 1, 2, 3 and 4 in FIG. 3 represent serial numbers of uplink synchronous HARQ processes of the same UE respectively. As can be apparent, no resource confliction arises between a packet retransmitted for the second time of the process 1 and a newly transmitted packet of the process 2 and between a packet retransmitted for the second time and a newly transmitted packet of other processes.
FIG. 4 illustrates configured values of delta and available resources for HARQ packets in the case of TDD Configuration 2, where both values of delta corresponding to the uplink HARQ processes 1 and 2 are 5, D represents a downlink (DL) sub-frame, U represents an uplink (UL) sub-frame, and S represents a special sub-frame. Both retransmission intervals of the uplink HARQ processes 1 and 2 are 10 ms, then in a 40 ms frame, resources of UL sub-frames 3, 13, 23 and 33 are available to the HARQ process 1 and resources of UL sub-frames 8, 18, 28 and 38 are available to the HARQ process 2.
A drawback of the foregoing solution lies in that if the value of delta corresponding to any semi-persistent scheduling in the multi-periodicity mode is 5 ms in the case of TDD Configuration 2, T1 is as follows.T1=SPS periodicity+delta=20+5=25 ms
For this configuration, there are eight uplink sub-frames in a 40 ms frame, only six of which are available to semi-persistent scheduling in the multi-periodicity mode. As illustrated in FIG. 5, in a 40 ms frame, resources corresponding to the uplink sub-frames 3 and 28 are allocated from semi-persistent scheduling in the multi-periodicity mode of a UE A (that is, the uplink sub-frames 3 and 28 are occupied respectively by two uplink HARQ processes of the UE A), likewise, resources corresponding to the uplink sub-frames 8 and 33 are allocated from semi-persistent scheduling in the multi-periodicity mode of a UE B, and resources corresponding to the uplink sub-frames 13 and 38 are allocated from semi-persistent scheduling in the multi-periodicity mode of a UE C. As can be apparent, the uplink sub-frames 18 and 23 are unavailable to semi-persistent scheduling in the multi-periodicity mode and consequently the resources are underused. If the uplink sub-frames 18 and 23 are used for semi-persistent scheduling in the multi-periodicity mode, the resources of these two uplink sub-frames have to be scheduled dynamically for transmission of a newly transmitted packet of user data, thus resulting in an excessive overhead of scheduling, which may cause an excessive restriction on uplink transmission and degrade the performance of a system. Likewise, the forgoing problem may also exist if the value of delta corresponding to any semi-persistent scheduling in the multi-periodicity mode is −5 ms.